The present invention relates generally to controllers for a combustion system for a gas turbine. In particular, the invention relates to a combustor control for a Dry Low NOx (DLN) combustor.
Industrial and power generation gas turbines have control systems with controllers that monitor and control their operation. These controllers govern the combustion system of the gas turbine. To minimize emissions of nitric-oxides (NOx), DLN combustion systems have been developed and are in use. Control scheduling algorithms are executed by the controller to operate DLN combustion systems. Conventional DLN algorithms receive as inputs measurements of the exhaust temperature of the turbine and of the actual operating compressor pressure ratio. DLN combustion systems typically rely solely on the turbine exhaust temperature and compressor pressure ratio to determine an operating condition, e.g., turbine exhaust temperature, of the gas turbine.
FIG. 1 depicts a gas turbine 10 having a compressor 12, a combustor 14, and a turbine 16 drivingly coupled to the compressor and a control system or controller 18. An inlet 20 to the compressor feeds ambient air and possibly injected water to the compressor. The inlet may have ducts, filters, screens and sound absorbing devices that each may contribute to a pressure loss of ambient air flowing through the inlet 20 into the inlet guide vanes 21 of the compressor. An exhaust duct 22 for the turbine directs combustion gases from the outlet of the turbine through ducts having, for example, emission control and sound absorbing devices. The turbine may drive a generator 24 that produces electrical power.
The operation of the gas turbine may be monitored by several sensors 26 detecting various conditions of the turbine, generator and environment. For example, temperature sensors may monitor ambient temperature surrounding the gas turbine, compressor discharge temperature, turbine exhaust gas temperature, and other temperature measurements of the gas stream through the gas turbine. Pressure sensors may monitor ambient pressure, and static and dynamic pressure levels at the compressor inlet and outlet, and turbine exhaust, as well as at other locations in the gas stream. Further, humidity sensors, e.g., wet and dry bulb thermometers, measure ambient humidity in the inlet duct of the compressor. The sensors 26 may also comprise flow sensors, speed sensors, flame detector sensors, valve position sensors, guide vane angle sensors, or the like that sense various parameters pertinent to the operation of gas turbine 10. As used herein, “parameters” and similar terms refer to items that can be used to define the operating conditions of turbine, such as temperatures, pressures, and flows at defined locations in the turbine that can be used to represent a given turbine operating condition.
A fuel control system 28 regulates the fuel flowing from a fuel supply to the combustor 14, the split between the fuel flowing into various nozzles and the fuel mixed with air before flowing into the combustion zone, and may select the type of fuel for the combustor. The fuel control system may be a separate unit or may be a component of a larger controller 18.
The controller may be a General Electric SPEEDTRONIC™ Gas Turbine Control System. The controller 18 may be a computer system having a processor(s) that executes programs to control the operation of the gas turbine using sensor inputs and instructions from human operators. The programs executed by the controller 18 may include scheduling algorithms for regulating fuel flow to the combustor 14. The commands generated by the controller cause actuators on the gas turbine to, for example, adjust valves between the fuel supply and combustors that regulate the flow and type of fuel, inlet guide vanes 21 on the compressor, and other control settings on the gas turbine.
The controller 18 regulates the gas turbine based, in part, on algorithms stored in computer memory of the controller. These algorithms enable the controller 18 to maintain the NOx and carbon monoxide (CO) emissions in the turbine exhaust to within certain predefined limits, and to maintain the combustor firing temperature to within predefined temperature limits. The algorithms include parameters for current compressor pressure ratio, compressor discharge temperature, ambient specific humidity, inlet pressure loss and turbine exhaust back pressure.
The combustor 14 may be a DLN combustion system. The control system 18 may be programmed and modified to control the DLN combustion system.
Turbine operating temperature and reference turbine operating temperature are important parameters in the control of a gas turbine operation. U.S. Pat. No. 7,100,357 by Morgan et al. described a system for controlling gas turbine by adjusting a target reference exhaust temperature that included a number of environmental factors incorporated in algorithms for calculating a reference temperature for turbine exhaust. The algorithms establish a limiting turbine exhaust temperature based on a NOx emission limiting algorithm, a CO emission limiting algorithm, a target turbine firing temperature algorithm, and a target turbine firing temperature limiting algorithm. The process may be used to maintain turbine emissions and firing temperature at or below target level, especially as ambient conditions and turbine operating parameter vary. The controller adjusts the fuel control to achieve the target turbine exhaust temperature. This algorithm is known as corrected parameter control (CPC).
Various normal transient operating conditions can result in a temporary difference between reference turbine operating temperature and actual turbine operating temperature. One example is when unloading a unit, the reference exhaust temperature is usually higher than the actual temperature because fuel is decreased first. Then, inlet guide vanes react to the error of actual versus reference temperature, but not to the decrease in fuel to hold firing temperature.
Unfortunately the inlet guide vanes are controlled using turbine exhaust thermocouples, and a known lag exists within the turbine exhaust thermocouples. By the time the turbine exhaust thermocouples register the lower temperature, fuel has continued to decrease. This results in the inlet guide vanes always “trailing” fuel while unloading, creating an under-fired condition.
Typically, significant margin exists on combustion systems in that the under-fire has no significant negative impact. However on advanced ultra low emissions combustion systems, the margins are much tighter. Transient under-fire can result in combustion dynamics or a loss of flame. Combustion dynamics within the combustor are known to damage hardware. Loss of flame in a combustion can creates high spreads, and the plugs are fired returning to Lean Lean, a high emissions mode of operation. A unit trip can also occur on high spreads.
Accordingly, new control algorithms are required to identify and transiently position the gas turbine unit to prevent combustion dynamics or loss of flame.